diploblastic

A diploblastic organism has a body cavity made of two cellular layers – an ectoderm
and an endoderm. The cnidarians are diploblastic, whereas all other metazoans are triploblastic. In small, simple cnidarians, such as Hydra,
the diploblastic condition is clear, but it is obscured in large jellyfish
and sea anemones. In diploblastic animals, however, unlike triploblastic ones, no organs develop from the middle layer, but only from the ectoderm
and endoderm.